In entrained-flow gasification, the feedstock to be gasified is held in a system called the feed system. From a reservoir vessel, also referred to as the feeder or fluidization vessel, and subject to a pressure for example of 4 MPa, the dustlike fuel (e.g., coal dust) is supplied via conveying lines to the burner. These conveying lines connect vertically from above into the bottom region of the reservoir vessel. The fuel is therefore conveyed from bottom to top. Stable, pulsation-free fuel dust conveying via the conveying line is critical to the optimum operation of the gasification plant. Stable conveying requires the bed of fuel to have an optimum density, known as the fluidization density, before entry into the conveying lines. If this requirement is not met, there may be severe disruptions to conveying.
As already outlined above, the attainment of an optimum fluidization density is a fundamental requirement for optimum, disruption-free fuel conveying. The fluidization density is set by way of the uniform addition of gas known as fluidizing gas, which may take the form of an inert gas such as nitrogen or carbon dioxide, for example, to the reservoir vessel from below. The addition of this gas leads to a reduction in the bulk density of the fuel. This means that more or less fluidizing gas is supplied, according to the requisite fluidization density. Uniform fluidization of the fuel over the entire area of the vessel base is critical to disruption-free conveying of the fuel drawn off from this fluidized layer. The uniformly distributed supply of the fluidizing gas has to date been ensured by means of a plate made of synthetic-resin-bonded pebble filter.
By virtue of its porosity, this material presents a very low flow resistance. At the same time, the porosity is correspondingly low, but is also selected to be small, in order to prevent fuel particles penetrating the filter and/or the gas supply line. In view of the physical properties of the sand filter, the maximum differential pressure is limited to levels permissible correspondingly by this element and/or by the maximum tensioning gas temperature. This limitation on the differential pressure and/or on the temperature does not normally pose any problems in plant operation. Exceedance of these parameters as a result of incorrect operating measures, such as shutoff of the fluidizing gas, which can lead to the maximum permissible differential pressure being exceeded, or the maximum permissible fluidizing gas temperature being exceeded, is accompanied by damage to the synthetic-resin-bonded pebble filter. Damage to this fluidizing plate, as it is called, may have the consequence of poor fluidizing gas distribution. This in turn leads to a situation in which the fluidization density required for stable conveying is unattainable at sites of relatively low fluidizing gas supply.
Exceeding the design parameters (differential pressure, temperature) can lead to damage to the pebble filter. Reducing the thickness of the filter material results in a reduction in the maximum permissible differential pressure. The maximum permissible differential pressure, however, is likewise an important design criterion for the element.